Pressure-sensitive tapes are virtually ubiquitous in the home and workplace. In its simplest configuration, a pressure-sensitive tape comprises an adhesive and a backing, and the overall construction is tacky at the use temperature and adheres to a variety of substrates using only moderate pressure to form the bond. In this fashion, pressure-sensitive tapes constitute a complete, self-contained bonding system.
According to the Pressure-Sensitive Tape Council, pressure-sensitive adhesives (PSAs) are known to possess properties including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength to be removed cleanly from the adherend. Materials that have been found to function well as PSAs include polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear holding power. PSAs are characterized by being normally tacky at room temperature (e.g., 20° C.). PSAs do not embrace compositions merely because they are sticky or adhere to a surface.
These requirements are assessed generally by means of tests which are designed to individually measure tack, adhesion (peel strength), and cohesion (shear holding power), as noted in A. V. Pocius in Adhesion and Adhesives Technology: An Introduction, 2nd Ed., Hanser Gardner Publication, Cincinnati, Ohio, 2002. These measurements taken together constitute the balance of properties often used to characterize a PSA.
With broadened use of pressure-sensitive tapes over the years, performance requirements have become more demanding. Shear holding capability, for example, which originally was intended for applications supporting modest loads at room temperature, has now increased substantially for many applications in terms of operating temperature and load. So-called high performance pressure-sensitive tapes are those capable of supporting loads at elevated temperatures for 10,000 minutes. Increased shear holding capability has generally been accomplished by crosslinking the PSA, although considerable care must be exercised so that high levels of tack and adhesion are retained in order to retain the aforementioned balance of properties.
There are a wide variety of pressure sensitive adhesive (PSA) materials available today that include natural crude or synthetic rubbers, block copolymers, and acrylic ester based polymeric compositions. Acrylic ester PSAs in particular have been the focus of a great deal of development over the last half century as the performance demands for PSAs have increased. Acrylic based PSAs may be closely tailored to provide a number of desired attributes such as elasticity, tackiness, transparency, resistance to oxidation and sunlight, etc., as well as have the necessary degree of adhesion and cohesion for demanding tape applications.
Central to all PSAs is a desired balance of adhesion and cohesion that is often achieved by optimizing the physical properties of the acrylic elastomer, such as glass transition temperature and modulus. For example, if the glass transition temperature (Tg) or modulus of the elastomer is too high and above the Dahlquist criterion for tack (storage modulus of 3×106 dynes/cm2 at room temperature and oscillation frequency of 1 Hz), the material will not be tacky and is not useful by itself as a PSA material. Often in this case, low molecular weight, high Tg resin polymers (tackifiers) or low molecular weight, low Tg polymers (plasticizers) are often used to modulate the Tg and modulus into an optimal PSA range.
Acrylic ester PSAs of today are typically an elastomeric polymer comprised primarily of a low Tg non-polar monomer, as well as a small amount of polar acrylic monomer such as acrylic acid. Two widely used low Tg acrylates in PSAs are 2-ethylhexyl acrylate (EHA) and isooctyl acrylate (IOA), each providing an alkyl chain of eight carbon atoms (C8). Longer or shorter alkyl chains have a number of disadvantages in terms of PSA performance. For example, shorter alkyl chain (e.g. butylacrylate-C4) will significantly increase both the Tg and modulus of the elastomer, possibly increasing the room temperature storage modulus above 3×106 dynes/cm2. Alternatively, longer alkyl chains (e.g. lauryl acrylate-C12) can lead to crystalline groups within the polymer that will also significantly reduce it degree of tack.